Remove python2 support from main version

[corentinbettiol]

We have Django Check SEO < 0.6 that's still compatible with python2 (see the branch python2).

So now I think we can remove support for python 2 and django < 2.2 from the package.

[dorian-adams]

Hi @corentinbettiol, I opened a PR that I believe addresses this. I mainly removed the UTF-8 encoding headers and any __future__ imports related to Python 2 compatibility. I also updated launch_tests to only use Python 3 and revised any associated documentation. If any adjustments are required, feel free to let me know! Thanks!